1. Field of the Invention
The present invention relates to an improvement in multistep emulsion/suspension polymerization techniques for forming polymeric materials.
2. Description of the Prior Art
A variety of multistep polymerization procedures involving one or more initial emulsion and one or more later suspension polymerization steps are known. The use of the terminology "emulsion/suspension polymerization" in describing the present invention is intended to relate to this general type of polymerization procedure. Thus, in U.S. Pat. No. 3,657,172 to Ruth E. Gallagher et al., rubber-containing interpolymers are formed by suspension polymerizing from about 40% to about 99%, by weight of the interpolymer, of vinyl monomer in the presence of a previously formed aqueous emulsion of particles comprising from about 0.5% to about 50%, by weight of the interpolymer, of a hard inner core of a polymer having a glass transition temperature (Tg) above about 25.degree. C. and from about 0.5% to about 59%, by weight of the interpolymer, of an outer layer comprising a crosslinked rubber having a Tg of less than about 25.degree. C. U.S. Pat. No. 3,832,318 to Ruth E. Gallagher et al. describes the formation of rubber-containing interpolymers which are prepared by suspension polymerizing from about 40% to about 98%, by weight of the interpolymer, of a vinyl monomer in the presence of from about 2% to about 60%, by weight of the interpolymer, of particles of a cross-linked acrylate rubber having a Tg of less than about 25.degree. C. A more recent example of such multistep emulsion/suspension polymerization procedures is described in U.S. Pat. No. 3,944,631 to A. J. Yu et al. which, in one embodiment, describes the suspension polymerization of from about 15% to about 90%, by weight of the interpolymer, of styrene and acrylonitrile in the presence of an aqueous latex of interpolymer particles comprising from about 5 to about 50%, by weight of the interpolymer, of a crosslinked (meth)acrylate component and from about 5% to about 35%, by weight of the interpolymer, of a crosslinked styrene-acrylonitrile component.
In the above types of processes, an emulsifying agent is used in the initial emulsion polymerization step or steps to form a latex of polymer particles. This emulsion polymerization portion of the process is performed using conventional emulsion polymerization techniques. The latex, or the emulsion resin particles isolated therefrom, is then used as one of the ingredients during one or more later, conventional suspension polymerization procedures. The presence of residual emulsifying agent in either the latex, or the product isolated therefrom, however, has a tendency to interfere with the dispersing ability of the suspending agent used in the suspension polymerization step or steps. This often leads to a certain degree of polymer flocculation during the suspension polymerization step or steps which can be especially deleterious in plant scale polymerizations where the reaction mixture needs to be pumped through piping during commercial operations. The presence of polymer flocculation tends to interfere with such pumping operations.